1. Field of the Invention
This invention pertains to electrolysis with emphasis on treatment of drinking water. It is an electrolytic process for water treatment utilizing electrical energy directly applied to the water being treated to generate halogen ions.
2. Description
The carcinogenic effects of disinfection byproducts (DBP) in chlorinated municipal drinking water supply prompted this invention. The presence of Natural Organic Matter (NOM as Total Organic Carbon—TOC) and bromide (mainly from estuaries) in raw water, commonly known as DBP precursors, leads to the formation of carcinogenic halogenated organic contaminants in drinking water upon subsequent chlorination. The brominated species in this class of contaminants have been shown to have significantly higher health effects compared to non-brominated species. This invention substantially reduces the amount of bromide available to form brominated disinfection byproducts.
Since the discovery of chlorination byproducts in drinking water in 1974, numerous toxicological studies have been conducted that show some DBPs to be carcinogenic and/or cause reproductive or developmental effects in laboratory animals. Additionally, exposure to high levels of disinfectants over long periods of time may cause health problems, including damage to blood and kidneys. While many of these studies have been conducted at high doses, the weight-of-evidence indicates that DBPs present a potential public health problem that must be addressed. USEPA Stage 1 DBP Rule limits Total Trihalomethanes (TTHM) to 80 μg/L Running Annual Average system-wide but Stage 2 will extend that limit to individual sample locations. Surface water sources that have links to estuaries or groundwater wells subject to influence of the sea exacerbate the tendency to form DBPs upon subsequent chlorination because of the presence of bromide in the water. These salt water intrusions result in significant levels of bromide which plays an important role in the relative concentrations of disinfection by product (DBP) species formed. Several researchers have reported that speciation shifted to the bromine-substituted THMs as a function of bromide concentration when all other parameters were held constant. Under conditions of high NOM and low bromide concentrations, chlorine-substituted byproducts predominated, especially during longer reaction times as the original bromide was consumed. In the presence of chlorine and precursor, as much as 50% of the bromide ion can become incorporated into the trihalomethane (THM) species bromodichloromethane, dibromochloromethane, and bromoform; this efficiency of bromide incorporation implies that 100 μg/L of bromide may result in up to 50 μg/L of THM-bound bromine (THM-Br). A reduction in bromide concentration will have a significant impact on the concentration and speciation of formed TTHM. Reverse osmosis and nanofiltration can reject 90% and 50% bromide respectively, but need residual handling capabilities and have low water recovery. It is also within the realm of possibilities that in the future, beyond Stage 2 DBP Rule, USEPA may set MCLs for individual DBP species that will warrant reduction in bromide concentration in the source water.
Chloramine has been employed as alternative disinfectant by many utilities to minimize the level of TTHMs formed from treating water with high concentration of precursors, however recent developments have implicated chloramine in the formation of nitrogenous DBPs like NDMA, cyanogen chloride and cyanogen bromide. Also, nitrification and the resulting odor is a common distribution system problem associated with chloramination.
Granular Activated Carbon (GAC) is effective in removing the main DBP precursor, TOC, from raw water but the replacement frequency in order to meet the DBP rule may make the cost of the process prohibitive especially when bromide is present in concentration above 100 μg/L. Electrolysis using silver electrodes and/or DSA to reduce bromide concentration in synergy with enhanced coagulation and a technology to reduce TOC will provide another option to achieve the treated water goal—less than 80 μg/L TTHM in the distribution system after 3 days—at a competitive cost.